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          <h1 class="post-title" itemprop="name headline">基于Java的LRU算法实现数据库字典缓存</h1>
        

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        <p>摘要：<br>    记一次解决项目问题，实现的LRU算法。</p>
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class="line">182</span><br><span class="line">183</span><br></pre></td><td class="code"><pre><span class="line">import java.util.HashMap;</span><br><span class="line"></span><br><span class="line">/**</span><br><span class="line"> * @author vlsion</span><br><span class="line"> */</span><br><span class="line">public class DicCacheUtil&lt;K, V&gt; &#123;</span><br><span class="line">    /**</span><br><span class="line">     * 定义初识化缓存长度即为HashMap容量</span><br><span class="line">     */</span><br><span class="line">    private final int MAX_SIZE;</span><br><span class="line"></span><br><span class="line">    /**</span><br><span class="line">     * 缓存中最新的数据</span><br><span class="line">     */</span><br><span class="line">    private Entry&lt;K, V&gt; start;</span><br><span class="line">    /**</span><br><span class="line">     * map中就老的数据</span><br><span class="line">     */</span><br><span class="line">    private Entry&lt;K, V&gt; end;</span><br><span class="line"></span><br><span class="line">    /**</span><br><span class="line">     * 缓存存放map</span><br><span class="line">     */</span><br><span class="line">    private HashMap&lt;K, Entry&lt;K, V&gt;&gt; cache;</span><br><span class="line"></span><br><span class="line">    /**</span><br><span class="line">     * 初始化缓存</span><br><span class="line">     *</span><br><span class="line">     * @param max_size</span><br><span class="line">     */</span><br><span class="line">    public DicCacheUtil(int max_size) &#123;</span><br><span class="line">        MAX_SIZE = max_size;</span><br><span class="line">        cache = new HashMap&lt;&gt;();</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    /**</span><br><span class="line">     * 定义缓存中的数据格式</span><br><span class="line">     * 其中pre为当前数据前面数据的key</span><br><span class="line">     * next为当前数据后面的key</span><br><span class="line">     * 通过pre 和 next 将缓存维护成链表形式的</span><br><span class="line">     *</span><br><span class="line">     * @param &lt;K&gt;</span><br><span class="line">     * @param &lt;V&gt;</span><br><span class="line">     */</span><br><span class="line">    public static class Entry&lt;K, V&gt; &#123;</span><br><span class="line">        K key;</span><br><span class="line">        V value;</span><br><span class="line">        Entry&lt;K, V&gt; pre;</span><br><span class="line">        Entry&lt;K, V&gt; next;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    /**</span><br><span class="line">     * 往缓存中添加数据时首先会去判断，添加的这个数据缓存中存不存在，</span><br><span class="line">     * 如果不存在：首先判断缓存的容量是否达到最大值，如果达到最大值，那么取缓存中最老的数据end将其移除，移除之后的数据再进行整理。整理过程看removeEnd方法注释。</span><br><span class="line">     * 然后将本次的数据整理(key=key,value=value)整理的数据还需要把链表的前后关联关系整理出来，之后将数据放入缓存中。</span><br><span class="line">     * &lt;p&gt;</span><br><span class="line">     * 如果存在的话，直接改变链表的逻辑关系具体看changeStart注解</span><br><span class="line">     *</span><br><span class="line">     * @param key</span><br><span class="line">     * @param value</span><br><span class="line">     */</span><br><span class="line">    public void put(K key, V value) &#123;</span><br><span class="line">        Entry&lt;K, V&gt; entry = cache.get(key);</span><br><span class="line">        if (entry == null) &#123;</span><br><span class="line">            if (cache.size() &gt;= MAX_SIZE) &#123;</span><br><span class="line">                cache.remove(end.key);</span><br><span class="line">                removeEnd();</span><br><span class="line">            &#125;</span><br><span class="line">            entry = new Entry&lt;&gt;();</span><br><span class="line">            entry.key = key;</span><br><span class="line">            entry.value = value;</span><br><span class="line">            changeStart(entry);</span><br><span class="line">            cache.put(key, entry);</span><br><span class="line"></span><br><span class="line">        &#125; else &#123;</span><br><span class="line">            entry.value = value;</span><br><span class="line">            changeStart(entry);</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    public V get(K key) &#123;</span><br><span class="line">        Entry&lt;K, V&gt; entry = cache.get(key);</span><br><span class="line">        if (entry == null) &#123;</span><br><span class="line">            return null;</span><br><span class="line">        &#125;</span><br><span class="line">        changeStart(entry);</span><br><span class="line">        return entry.value;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    /**</span><br><span class="line">     * 移除缓存中的最老的数据首先判断是否存在end，end存在的情况下，把end前面的数据拿出来当做最老的数据。</span><br><span class="line">     * 因为我们链表定义的是最老的在末尾，最新的在最前，中间也是依次排列的。所以才有上面的把end之前的拿出来当最老的。</span><br><span class="line">     */</span><br><span class="line">    public void removeEnd() &#123;</span><br><span class="line">        if (end != null) &#123;</span><br><span class="line">            Entry&lt;K, V&gt; prev = end.pre;</span><br><span class="line">            if (prev == null) &#123;</span><br><span class="line">                start = end = null;</span><br><span class="line">            &#125; else &#123;</span><br><span class="line">                end.pre = null;</span><br><span class="line">                end = prev;</span><br><span class="line">                end.next = null;</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    /**</span><br><span class="line">     * 这一部分比较绕；</span><br><span class="line">     * 分三步：第一，要插入的数据是不是最前面的一个。</span><br><span class="line">     * 最前面的一个，我们就直接返回。</span><br><span class="line">     * &lt;p&gt;</span><br><span class="line">     * 第二要插入的数据是不是最后面的一个。</span><br><span class="line">     * &lt;p&gt;</span><br><span class="line">     * 第三要插入的数据是在中间的。</span><br><span class="line">     * 中间的时候，将中间的拿走当做最新的数据后，之前中间的部分，空出来了需要把中间之前的和中间之后的连接点接上。</span><br><span class="line">     *</span><br><span class="line">     * @param entry</span><br><span class="line">     */</span><br><span class="line">    public void changeStart(Entry&lt;K, V&gt; entry) &#123;</span><br><span class="line">        if (entry == start) &#123;</span><br><span class="line">            return;</span><br><span class="line">        &#125;</span><br><span class="line">        if (entry == end) &#123;</span><br><span class="line">            Entry&lt;K, V&gt; prev = entry.pre;</span><br><span class="line">            if (prev != null) &#123;</span><br><span class="line">                end.pre = null;</span><br><span class="line">                end = prev;</span><br><span class="line">                end.next = null;</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">        if (start == null || end == null) &#123;</span><br><span class="line">            start = end = entry;</span><br><span class="line">            return;</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        //将中间的拿走当做最新的数据后，之前中间的部分空出来了,需要把中间之前的和中间之后的连接点接上。</span><br><span class="line"></span><br><span class="line">        //中间之前的下一个变成中间的下一个</span><br><span class="line">        if (entry.pre != null) &#123;</span><br><span class="line">            entry.pre.next = entry.next;</span><br><span class="line">        &#125;</span><br><span class="line">        //中间下一个的前一个变成之间的前一个</span><br><span class="line">        if (entry.next != null) &#123;</span><br><span class="line">            entry.next.pre = entry.pre;</span><br><span class="line">        &#125;</span><br><span class="line">        entry.next = start;</span><br><span class="line">        entry.pre = null;</span><br><span class="line">        start.pre = entry;</span><br><span class="line">        start = entry;</span><br><span class="line"></span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    @Override</span><br><span class="line">    public String toString() &#123;</span><br><span class="line">        StringBuilder stringBuilder = new StringBuilder();</span><br><span class="line">        Entry&lt;K, V&gt; entry = start;</span><br><span class="line">        while (entry != null) &#123;</span><br><span class="line">            stringBuilder.append(String.format(&quot;%s:%s &quot;, entry.key, entry.value));</span><br><span class="line">            entry = entry.next;</span><br><span class="line">        &#125;</span><br><span class="line">        return stringBuilder.toString();</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    public static void main(String[] args) &#123;</span><br><span class="line">        DicCacheUtil&lt;Integer, Integer&gt; dicCacheUtil = new DicCacheUtil&lt;&gt;(5);</span><br><span class="line">        dicCacheUtil.put(1, 1);</span><br><span class="line">        System.out.println(dicCacheUtil);</span><br><span class="line">        dicCacheUtil.put(2, 2);</span><br><span class="line">        System.out.println(dicCacheUtil);</span><br><span class="line">        dicCacheUtil.put(3, 3);</span><br><span class="line">        System.out.println(dicCacheUtil);</span><br><span class="line">        dicCacheUtil.get(1);</span><br><span class="line">        System.out.println(dicCacheUtil);</span><br><span class="line">        dicCacheUtil.put(4, 4);</span><br><span class="line">        System.out.println(dicCacheUtil);</span><br><span class="line">        dicCacheUtil.put(5, 5);</span><br><span class="line">        System.out.println(dicCacheUtil);</span><br><span class="line">        dicCacheUtil.put(6, 6);</span><br><span class="line">        System.out.println(dicCacheUtil);</span><br><span class="line">        dicCacheUtil.get(4);</span><br><span class="line">        System.out.println(dicCacheUtil);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>运行结果</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">1:1 </span><br><span class="line">2:2 1:1 </span><br><span class="line">3:3 2:2 1:1 </span><br><span class="line">1:1 3:3 2:2 </span><br><span class="line">4:4 1:1 3:3 2:2 </span><br><span class="line">5:5 4:4 1:1 3:3 2:2 </span><br><span class="line">6:6 5:5 4:4 1:1 3:3 </span><br><span class="line">4:4 6:6 5:5 1:1 3:3</span><br></pre></td></tr></table></figure>
      
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